Great attention has been paid to ceramic materials for their heat resistance, wear resistance, high-temperature strength and other advantages. However, ceramic materials are extremely difficult to mechanically work since they are hard and brittle. Thus most ceramic articles are prepared by sintering and precursor methods. In the sintering method, a ceramic material in powder form is pressed or otherwise molded into a desired shape and then fired for sintering. The precursor method is by melting an organic polymer as a ceramic precursor or dissolving it in a suitable solvent, molding the melt or solution into a desired shape, and then firing for converting the polymer into inorganic form. The precursor method is characterized by the potential manufacture of ceramic articles to a configuration which cannot be achieved with the powder sintering method, and especially adapted for the manufacture of fibers.
Among ceramics, SiC and Si.sub.3 N.sub.4 are of great interest for high-temperature performance, more particularly because of heat resistance and high-temperature strength for the former and thermal shock resistance and fracture toughness for the latter. Extensive research works have been made on their precursors. The silicon carbide and nitride ceramic materials are also considered useful as reinforcements for fiber-reinforced composite materials by taking advantage of their light weight, heat resistance, and high strength features. Thus integration of these reinforcements with plastics, metals and ceramics is also an important subject.
In the prior art, ceramic fibers are prepared by forming an organic silazane polymer through pyrolytic polymerization and converting the polymer as a precursor into ceramic fibers composed of SiC and Si.sub.3 N.sub.4 as described in U.S. Pat. No. 3,853,567 (Japanese Patent Publication No. 46995/1980). This method produces an organic silazane polymer by using a silazane compound resulting from a halosilane and an amine and heat polymerizing the compound at high temperatures in a Raschig ring packed column. This method has the following problems.
(1) Only limited reactants are available. The patent specification sets forth only methyltrichlorosilane and dimethyldichlorosilane as the halosilane and monomethylamine as the amine.
(2) This method produces an organic silazane polymer by passing a monomer or silazane compound through a column loaded with packings such as Raschig rings at a temperature as high as 200.degree. to 800.degree. C. This method allows the monomer to polymerize upon contact with the packings, but the extended contact with the packings can cause some polymers to convert into higher polymers. Such insoluble, infusible, highly polymerized solids will gradually accumulate in the column to clog the column, disturbing further continuation of polymerization reaction.
(3) The method based on a Raschig ring packed column allows a considerable amount of a crystalline by-product to form. The by-product precipitates and deposits on the gas phase-defining interior wall of the reactor, participating in reaction no longer. As a result, the end organic silazane polymer is obtained in a low yield of 36%.
For these drawbacks, the method of U.S. Pat. No. 3,853,576 is difficult to commercially effectively prepare an organic silazane polymer which is a ceramic precursor.
U.S. Pat. No. 3,892,583 discloses a method for preparing SiC--Si.sub.2 N.sub.4 ceramics by reacting various chlorosilane compounds with ammonia and firing the resulting silazane polymers. It is described that for a mixture of alkylhalosilanes RSiX.sub.3 and R.sub.3 SiX, the preferred mixing proportion is RSiX.sub.3 /R.sub.3 SiX=50-100 mol %/ 50-0 mol %. We have found, however, that if the mixing proportion of a trifunctional halosilane exceeds 50 mol %, there are formed larger amounts of silazane compounds which are insoluble in the solvent. Since the desired polymer is obtained in substantially reduced yields, this method is unsuitable for commercial manufacture.
The above-mentioned methods are difficult to manufacture high quality ceramics composed mainly of Si, C and N in a commercially advantageous manner. There is a desire for overcoming these problems.
Recently, there is an increasing need for application of ceramic precursors to ceramic coatings and binders. It is thus desired to reduce the cost of ceramic precursors although most ceramic precursors are less cost effective at present. There is also a desire for overcoming the economical problem.